For example, a so-called machined elbow formed by machining a material is known. However, even when a so-called 5-axis machining device or the like capable of performing processing at a higher level is used during a manufacturing process of the machined elbow, unmachined portions remain on an inner side surface of a target elbow in some cases. In order to solve the above-mentioned problem, for example, Patent Literature 1 discloses a method of manufacturing a machined elbow without unmachined portions by roughly processing a material to form a starting hole, revolving a side cutter, which has cutting edges having an arc in shape on an outer circumference thereof, along an inner side surface of a target elbow, and moving the side cutter in a spiral shape for cutting.